Device for controlling the terminal voltage of a current load



April 24, 1951 H. B. e. CASIMIR ET AL 2,549,959

DEVICE FOR CONTROLLING THE TERMINAL VOLTAGE OF A CURRENT LOAD Filed Feb. 1947 H. 5.6. CASIM IR 8. H. NIEUWDORP AGENT Patented Apr. 24, 1951 DEVICE FOR CONTROLLING THE TERMINAL VOLTAGE OF A CURRENT LOAD Hendrik Brugt Gerhard Casimir and Hendrik Nieuwdorp, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Application February 7, 1947, Serial No. 727,000 In the Netherlands April 27, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires April 27, 1964 V 7 Claims. 1

The invention relates to electrical devices for stabilizing the voltage across a load supplied by a fluctuating source.

For controlling the terminal voltage of a load it has heretofore been proposed that a discharge tube having a control grid should be connected in series with this current load, the voltage drop in this tube being controlled by means of its grid voltage. The present invention relates to a device in which this method is used. The control grid discharge tube connected in series with the current load is referred to hereinafter as the control tube in contradistinction from other discharge tubes to be mentioned hereinafter, which the device comprises or may comprise.

A particular technique for controlling the terminal voltage by means of a control tube is the switching-on and switching-off of the current, i, e. making a choice of two values of the terminal voltage, one of which is the zero value. A further technique using such a tube involves maintaining a constant terminal voltage with a variable supply voltage, it being possible to make this constant value adjustable. As an alternative, the terminal voltage may be any function of time, even a periodical function.

The invention provides a solution of the prob lem of establishing the required coupling if the grid circuit of the control tube has high voltage relatively to the means by which the grid voltage must be controlled (which generally will be at ground or approximately ground potential).

According toa prior suggestion use is made in such techniques of a transformer whose primary winding is connected to a voltage regulator and is insulated from the secondary winding against high voltage. This coupling is not suitable for high-speed control, at least if the voltage furnished by the transformer must be rectified and filtered.

According to the invention, which has for its object to ensure a high-speed control under all conditions, the control means have connected to them a magnet coil and the grid circuit of the control tube is connected to an electron discharge tube. This second discharge tube will be referred to hereinafter as the governing tube. The electron paths cf this governing tube are under the influence of the magnetic field of the magnet coil and its current intensity is controlled by means of this field.

It is known per se to control the electric current in a discharge tube by means of a magnetic field. For this purpose, use may be made of a tube of cylindrical shape, a cylindrical anode and 2 an incandescent cathode being arranged coaxiall within the tube. This tube is arranged in such manner that the lines of force of the magnetic field in the tube are parallel to its axis. For this purpose the tube may be placed inside the coil axially thereof.

In the device according to the invention there is a great diiference between the potential of the electrodes of the governing tube and that of the magnet coil. Nevertheless the current of the governing tube can be controlled readily and without delay by variation of the current generating the magnetic field. By adequately increasing the intensity of this current it is possible to achieve such a curvature of the electron paths that the grid voltage of the control tube is. given an extreme value. This may be thatat which the terminal voltage of the current load has the zero value or the maximum value and this depends on the manner in which the governing tube is connected to the grid circuit of the control tube.

Use may also be made of the property that in a tube in which a screen grid is arranged in the shadow space of a control grid the total flow of electrons decreases at an increasing intensity. of the magnetic field, but the screen grid current which is the difference between the total current and the anode current assumes a higher value, since, owing to the curvature of the path more electrons find their way to the rodsof the screen grid.

In a particular embodiment of the device. according to the invention such a screen grid tube, preferably a pentode, is used as the governing tube, the grid voltage of the control tube, or at least a component thereof being obtained from a resistance which connects anode and screen grid of the governing tube. With increasingstrength of the screen grid current, as a result of the amplification of the magnetic field, the voltage across this resistance increases so that the grid voltage of the control tube varies.

At the same time, however, the intensity of the total flow of electrons decreases and this counter.- act the increase of the screen grid current. In an improved form of construction of this device provision is therefore made, on the cathode side, of a resistance which is connected in series with the governing tube and which is included in the grid circuit of this tube. The voltage drop in this resistance thus supplies a negative component to the control grid voltage of the governing tube. This is compensated for by a positivecomponent furnished by a source of electromotive power included in the grid circuit. The sensitiveness to the current variations in the magnet coil is enhanced by this.

A device according to the invention, which particularly serves to cause the terminal voltage of the current load to fulfill a prescribed function at a variable supply voltage (such as in the case of supplying power by a condenser), comprises a third discharge tube included in the circuit of the magnet coil. This tube which is provided with a control grid, will be referred to as the master tube. Its grid voltage has a component which is proportional to the terminal voltage of the current load. This grid voltage component is transferred by resistances from the level of high voltage to that of the control means. When its grid voltage changes, the master tube causes the current in the magnet coil to vary and consequently the current of the governing tube varies. This is connected to the grid circuit of the control tube in such manner that the current variations of the governing tube counteract the voltage variations of the current load.

It often occurs that the source of supply current has its midpoint grounded, it being thus possible for the cathode of the master tube to be connected to this point. Thus, if the grid of the master tube is connected through a resistance to the cathode, through a similar resistance to the anode of the control tube and through a regualting resistance to ground, the voltage across this regulating resistance is proportional to the terminal voltage of the current load and can hence be used as a variable component of the grid voltage of the master tube. Connection of this resistance in series with a source of electromotive force which is oppositely directed to the voltage drop in the regulating resistance between grid and cathode of the master tube ensures a compensation of the variable component by a fixed component. The variations of the former bring about intensity variations of the coil cmrent.

If the control tube is connected on the negative side of the current load, the regulating resistance furnishes a negative grid voltage, which must consequently be compensated for by a positive comparison voltage. In this case the source of the coil current may also be used to furnish the required positive compensation voltage.

If the control tube is connected to the positive side of the current load, the regulating resistance furnishes a positive grid voltage and a negative compensation voltage and also a separate anode current source are required.

If the connection of the control means to the supply current source is not established at the midpoint, but in such manner that with respect to this connection the positive terminal of the current source has a potential pV and the negative a potential V, in order that a voltage across the regulating resistance may be obtained which is proportionate to the terminal voltage of the current load, the resistance connecting the grid of the master tube to the anode of the control tube must be the p-fold of the resistance which connects it to the cathode of this tube.

The current which passes through the regulating resistance is under the influence of the compensating voltage, but if the latter is very low compared with V and 12V, the proportionality aimed at is achieved to a sufficient approximaion.

The regulating resistance permits of the terminal voltage being given any value within definite limits, a value which, also again within definite limits, is independent of the voltage of the supply current source.

To break the current, the grid of the master tube may be connected to the cathode by means of a switch via a source of negative grid voltage. By opening this switch the current is started. The compensation voltage being low and the voltage drop across the regulating resistance being of the same order of magnitude, merely a low voltage being also required for disrupting the current across the grid of the master tube, only low voltages occurring in the control member of the control resistance and the switch for making and breaking the current.

In order that the invention may be clearly understood and readily carried into effect, it will now be set out more fully with reference to the accompanying drawing, which represents, by way of example, some few circuit diagrams of forms of construction of devices according to the invention.

Fig. 1 shows a simple circuit-arrangement, in which the current can be made and broken in the manner according to the invention and hence only the first particular technique for voltage control mentioned supra occurs in this case.

Fig. 2 shows a circuit diagram, in which the governing tube is connected otherwise and in which this tube permits of giving the terminal voltage of the current load differing values.

Fig. 3 is the circuit diagram of an apparatus,

' in which a governing tube and a magnet coil enable the current of a load in the form of an X-ray tube to be made and broken and the voltage of the tube to be adjusted at a given value which it maintains, even if an appreciable change of the voltage of the supply source occurs.

Fig. 4 is the circuit diagram of similar apparatus in which the governing tube is connected in a different manner, a pentode acting as such governing tube. In addition, the control valve is located on the positive side of the X-ray tube, and not on the negative side, as in Fig. 3.

Referring to Fig. l, a current load I, a control tube 2 and a governing tube 3 are connected in series in the said order with a direct current source 4. The cathode of the control tube is connected to the anode of the governing tube; the grid of the control tube is connected to the oathode of the governing tube.

The governing tube is surrounded by a coil 5 which is connected to a current source 6. A switch I is inserted in the current path of the coil. When this current flows, the electron paths in the tube 3 which are intersected at right angles by the lines of force of the magnetic field of the coil 5, are curved and the resistance of the tube 3 and hence also that part of the voltage which it absorbs are increased. The voltage across the tube 3 acts as a negative grid voltage on the tube 2. At sufiicient intensity of the current in the coil 5, the grid of the tube 2 becomes negative to such an extent that the current is substantially reduced to zero. The switch 1 by opening allows the current to flow to the load and by closing breaks said flow. Even though the electrodes of the governing tube 3 and the cathode of the control tube 2 on the one hand and the means operative to control the grid voltage of the control tube, are in the grid circuit of the control tube 2, is a high potential difference, in the circuit of coil 5, and there is no objection to coupling the control means with the grid circuit of the control tube. The risk of spark discharges transformer oil.

occurring between the coil 5 and the tube 3 can be obviated by means of a suitable insulating means, for example an insulating liquid such as If desired the circuit of the coil may be grounded, as is designated at 8. Consequently, in the manner described the current of a circuit in which a high voltage is eiiective or which as a whole exhibits a high potential difference relatively to earth can be made and broken by means of a switch connected to ground.

The circuit shown in Fig. 2 includes a separate current source 9 for the governing tube 3 connected to it in series with a resistance Ill. The

voltage drop in this resistance acts as a negative grid bias on thecontrol tube 2, the passage of the current in the coil 5 (which as before should be assumed to surround the tube 3) consequently having an effect opposite to that shown in Fig. 1. When the current starts to flow, the current in the tube 3 weakens and the voltage across the resistance l decreases. As a result,the tube 2 which so far, due to its negative grid bias, disrupted the current, will allow the current to pass. Consequently upon the current in the coil being broken, the current of the load is also switched out.

The terminal voltage of the load depends on the resistance of the control tube 2 and hence on the intensity of the anode current of the tube 3. The latter, in turn, depends on the field strength of the magnetic field of the coil 5, consequently on the intensity of the current which is sent through the coil 5. Altering this current strength consequently permits of controlling the terminal voltage of the load For this purpose a regulating resistance II is connected in series with the coil 5.

A given value of the current strength of the coil circuit is associated with a given value of the terminal voltage of the load A measuring instrument I2 can therefore be calibrated in units of this voltage.

Referring to Fig. 3, I3 and M designate condensers which are charged by the intermediary of rectifiers I5 and I6 in a Greinacher circuitarrangement. A transformer serves as the source of charging current. The current load is an X-ray tube IS, with which a control tube I9 is connected in series on the negative side.

The negative coating of the condenser l3 and the positive one of the condenser I4 are grounded. This connection may be referred to as the zero point of the circuit. The positive coating of the condenser I3 and the negative one of the condenser l4 have a high voltage. It is impossible for the circuit of the X-ray tube to include a switch adapted to be operated manually. This is impossible even at the zero point of the circuit, because a high voltage would occur when the switch is opened.

The invention obviates this difficulty in the following manner. The zero point of the circuit is connected to the cathode of a triode 20, the master tube. This tube is included in a circuit which comprises a current source 2| and a magnet coil 22. Grid and cathode of the tube 23 are interconnected by a source of negative grid bias 23 and a switch 24, connected in series therewith. The coil 22 surrounds the governing tube which is connected in a manner similar to the governing tube 3 of Figure 2. The negative grid bias of the control tube I9 is obtained from the resistance 26.

When the switch 24 is closed, the grid of the tube 20 is negative and the coil 22 is dead. In this case the grid of the tube I9 is also negative and no current flows in the X-ray tube |8. When the switch 24 is opened, a current flows in the coil 22' and hence also in the X-ray tube |8. There is no risk of a high voltage occurring, because one of the terminals of the switch 24 is directly grounded, and the other is grounded via a source of low voltage 2| and a low regulating resistance 21.

The grid of the master tube 20 is connected via a high resistance 28 to the anode and via a high resistance 29 to the cathode of the control tube. If the resistance of the X-ray tube is referred to as 11, that of the control tube as 12 and the values of the resistances 28, 29 and 21 as r3, 1'4 and m respectively and if the voltages of the condensers 3 and 4 are designated V1 and V2 and that of the current source 2| V3, the following equations can be formulated:

i1, is, 2'4 and is designating respectively the currents in the X-ray tube l8 and the currents in the resistances 28, 29 and 21.

If the ratio between the resistances m and 1'4 is chosen to be equal to that between the voltages V1 and V2, so that if V1 is assumed to equal pV2, r3=pm (p will generally be :1) it is possible by elimination of i3 and 2'4 from the above equations to derive:

Now, iiri is the terminal voltage of the X-ray tube V13 and im. is the voltage drop V21 across the variable resistance 21. It follows that:

The values V1 and V2 do not occur in this equation. A variation in the supply voltage V=V1+V2 does not change the ratio between V18 and V21. Variations in the terminal voltage of the X-ray tube do bring about changes in the voltage V27. The latter acts as a negative component of the grid voltage on the master tube 20 and is compensated by the voltage of the anode current source 2| of this tube.

Variations in the terminal voltage of the X- ray tube are consequently attended by proportional variations of this grid voltage and also cause the current in the tube 2|), and hence the current in the tube 22, to vary.

As regards the reaction on the circuit of the X-ray tube Hi, there is the following performance: If the voltage of the condensers l3 and I5 decreases, the voltage across, the X-ray tube tends to drop and hence the voltage V27 tends to drop. The potential of the grid of the tube 20 tends to rise and the current in coil 22 to increase. An intensification of the field of the coil 22 involves weakening of the current of the governing tube 25 and hence a decrease of the negative grid bias of the control tube |9 and a cutting down of the resistance of this tube.

The voltage V27 becomes adjusted approximately to the amount of the compensation voltage Va consequent upon the action of the tubes, 23, 25 and |3. Varying the resistance 1'5 therefore permits of controlling the voltage of the X- ray tube, but within given limits this voltage constantly remains at the value adjusted at the resistance 21, even if the voltage of the condensers decreases.

Referring to Fig. 4, the anode of the X-ray' tube 18 is connected to the cathode of the control tube IS. The master tube 22 is connected in a manner similar to that shown in Fig. 3 and the terminal voltage of the X-ray tube is transmitted to the grid circuit of the tube 20 in an identical manner by resistances Z6 and 21.

The regulating resistance 2'! has current passing through it in a direction opposite to that of the device shown in Fig. 3. Thus, the voltage loss in this resistance supplies a positive grid voltage component to the master tube 20. Consequently, the source of the comparison voltage must have its positive terminal connected to the zero point of the circuit, because it has now to furnish a negative compensation voltage. In addition, provision must be made of a separate source of current 30 to furnish the coil current. Furthermore, the governing tube must be connected in a different manner, because with a decreasing voltage of the X-ray tube the grid potential of tube 20 drops and the coil current decreases.

In the example use is made of a pentode 3| to constitute the governing tube. The screen grid of this tube is placed in the shadow space of the control grid. This is understood to mean such an arrangement that the rods of the screen grid 32, viewed from the cathode 33, are hidden behind those of the control grid 34, so that no electrons at all or only a few electrons emitted by the cathode reach the screen grid. The secondary electrons emitted by the anode 35 do not reach the screen grid either, because the screen grid and the anode have arranged between them a grid 36 connected to the cathode which repels the secondary electrons to the anode. On the magnetic field being developed, the electron paths are curved and a greater or lesser portion of the electrons impinge on the screen grid.

In the device shown in Fig. 4 use is made of the property that with a decreasing coil current the portion of the electrons which are conducted away by the screen grid of the pentode 3i decreases. The anode of the tube 3i is connected direct, the screen grid 32 via a resistance 31 to the positive terminal of the anode current source 38. With a decreasing screen grid current the voltage between these electrodes drops. The resistance 31 connects the cathode of the control tube 19 to the grid of this tube, so that the negative grid bias which it procures to the control tube decreases with a decreasing coil current.

The sensitiveness of the control at constant voltage can be further increased by the control grid 34 of the tube 3| being given a voltage consisting of a fixed positive component obtained from the current source 38 and a negative component proportionate to the current which is furnished by a resistance 39. In this case use is made of the property that with a decreasing coil current the sum of the anode current and the screen grid current increases, though the latter, considered by itself, decreases. With an increasing current in the resistance 39, the potential of the grid 34 drops and the resistance of the tube consequently increases. This leads to a heavier decrease of the screen grid current and hence to an increased sensitiveness in a given range to small variations of the magnetic field.

Even for purposes other than with the device shown in Fig. 4 a pentode may be used to constitute the governing tube. If desired, an amplifier may be interconnected between the pentode and the control tube.

What we claim is:

1. Apparatus for stabilizing the voltage across a load supplied from a potential source subject to fluctuations comprising a control electron discharge tube interposed between said load and said potential source and including a grid, a grid bias circuit for said control tube, a governing electron discharge device including an electromagnetic coil for varying current flow therein, said device being connected in said grid bias circuit to Vary the bias on said grid in accordance with said current flow, a master electron discharge tube including a control electrode, a constant voltage source connected in series with said master tube across said coil, and means to apply a voltage to said control electrode proportional to the voltage across said load to vary the voltage across said coil and thereby the bias on said grid to an extent effecting compensation for fluctuations in said. potential source.

2. Apparatus for stabilizing the voltage across a load supplied from a potential source subject to fluctuations comprising a control electron discharge tube interposed between said load and said potential source and including a cathode, a grid and an anode, a grid bias source connected between the cathode and the grid of said control tube, a governing electron discharge device connected in series with said bias source and ineluding a cathode and an anode and an electromagnetic coil for varying current flow in said device, a resistance connected across said seriesconnected bias source and device for applying voltage to the anode of said device and bias voltage to the grid of said control tube in accordance with the current flow in said device, a master electron discharge tube including a control electrode, a constant voltage source connected in series with said master tube across said coil, and means to apply a voltage to said control electrode proportional to the voltage across said load to vary the voltage across said coil and thereby the bias on said grid to an extent effecting compensation for fluctuations in said potential source.

3. Apparatus, as set forth in claim 2, further including means for deriving a constant bias for the control electrode of said master tube from said constant voltage source.

4. Apparatus, as set forth in claim 2, further including a separate source of cut-off bias for said master tube, a control switch, and means to apply said cut-oil bias to the control electrode of said master tube in series with said switch.

5. Apparatus, as set forth in claim 2, wherein said voltage proportional to the voltage across said load is derived from a point in an impedance element having a relatively high value shunted across said control tube.

6. Apparatus for stabilizing the voltage across a load supplied from a potential source subject to fluctuations comprising a control electron discharge tube interposed between said load and said potential source and including a cathode, a grid and an anode, a governing electron discharge device including a cathode, a grid, 2. screen grid, an anode and an external electromagnetic coil for varying current flow in said device, an anode voltage source connected between the cathode and anode of said device,

said control electrode proportional to the volt- 10 age across said load to vary the voltage across said coil and thereby the bias of the grid of said control tube to an extent effecting compensation for fluctuations in said potential source.

7. Apparatus as set forth in claim 6 further including a separate source of cut-on" bias for said master tube, a control switch, and means to apply said cut-off bias to the control electrode of said master tube in series with said switch.

HENDRIK BRUGT GERHARD CASImR. HENDRIK NIEUWDORP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,604,364 Morrison Oct. 26, 1926 1,641,732 Bruckel Sept. 6, 1927 2,036,070 Morrison Mar. 31, 1936 2,227,353 Kuntke Dec. 31, 1940 2,264,051 Niemann Nov. 25, 1941 2,352,231 Stratton June 27, 1944 

